1. Field of the Invention
The present invention relates to an electrochemical device for use with various electronic devices. Particularly, the present invention relates to an electrochemical device in which compositions for use in an electrolytic solution and a seal are improved.
2. Description of the Related Art
Known electrochemical devices which require an electrolytic solution in order to drive such devices include an aluminum electrolytic capacitor, an electric double layer capacitor, a lithium battery, a lithium ion battery and an electrochromic device.
Referring to FIG. 2, a conventional aluminum electrolytic capacitor 60 will now be described as an exemplary electrochemical device. The aluminum electrolytic capacitor 60 includes: a capacitor element 51 as a main body; an electrolytic solution (not shown) which is impregnated in the capacitor element 51; a tubular metallic case 56 made of aluminum which has a bottom surface and accommodates the capacitor element 51; a seal 57 for sealing an opening of the case 56; and a cover 58 for covering the case 56.
The capacitor element 51 is formed by rolling up an anode foil 52, a cathode foil 53 and a separator 54 disposed between these foils. A pair of lead wires 55a and 55b are coupled to the anode foil 52 and the cathode foil 53, respectively. The lead wires 55a and 55b each include a rod-like terminal section and a solderable lead section for external connection. The terminal section includes a flat surface which is in contact with the anode foil 52 or the cathode foil 53.
An electrolytic solution used in such an aluminum electrolytic capacitor includes one which is obtained by dissolving an electrolyte such as an organic acid, an inorganic acid and salts thereof in a solvent such as .gamma.-butyrolactone or N,N-dimethylformamide. For example, those using a quaternary ammonium salt of a maleic acid or a citraconic acid (Japanese Patent Publication for Opposition No. 3-6646) and those using a quaternary ammonium salt of an aromatic carboxylic acid (Japanese Patent Publication for Opposition No. 3-8092) are known in the art.
As the seal material, an ethylene propylene copolymer and a butyl rubber, which are vulcanized with sulfur, are used. Known seal materials with improved heat stability include a butyl rubber which is vulcanized with an alkylphenol formalin resin (Japanese Laid-open Publication No. 62-276819).
As another exemplary electrochemical device, a conventional electric double layer capacitor will now be described. The electric double layer capacitor has a structure which is similar to that of the aluminum electrolytic capacitor illustrated in FIG. 2. An electrolytic solution used in such an electric double layer capacitor includes one which is obtained by dissolving an electrolyte such as an inorganic acid salt in an organic solvent such as propylene carbonate. For example, as an inorganic acid salt, a quaternary ammonium salt of perchloric acid or tetrafluoroboric acid are known in the art.
However, in such a conventional electrochemical device including the above aluminum electrolytic capacitor and the electric double layer capacitor, when a voltage, particularly a reverse voltage, is applied to the electrochemical device, an excessive amount of hydroxide ions may be generated by the electrolysis of the electrolytic solution. When such an electrochemical device is used for a long time under high temperature conditions, particularly, high temperature and high humidity conditions, the electrolytic solution often leaks through the seal. This leakage is due to an increase in the internal pressure of the electrochemical device and deterioration of the seal by the alkalinity of hydroxide ions.
In order to address such leakage problems, a seal made of a butyl rubber obtained by vulcanization with an alkylphenol formalin resin, vulcanization with a resin, or the like, has been proposed. Such a seal is generally more resistant to the deterioration by alkalis. The leakage of the electrolytic solution due to the excessive hydroxide ions is particularly suppressed when such a seal is used in combination with an electrolytic solution containing, as an electrolyte, a quaternary salt of a compound having an alkyl-substituted amidine group (see WO 95/15572).
However, such a combination is still insufficient for eliminating leakage during long-term use of an electrochemical device under high temperature conditions, particularly, high temperature and high humidity condition. Thus, it is desired to obtain seals having more reliable sealing properties.